Heat pump system and method of operating

ABSTRACT

A heat pump system operable in a cooling mode, a heating mode and a defrost mode includes a refrigerant compressor ( 20 ), a reversing valve ( 30 ), a first heat exchanger ( 40 ) and a second heat exchanger ( 50 ) disposed in a refrigerant circuit, and a primary expansion valve ( 45 ) disposed in the refrigerant circuit between said first heat exchanger ( 40 ) and said second heat exchanger ( 50 ); said reversing valve ( 30 ) is positionable in a first position for operation of said heat pump system in the cooling mode or defrost mode and is positionable in a second position for operation of said heat pump system in the heating mode; a refrigerant bypass circuit establishes a refrigerant flow path from the refrigerant circuit at a first location upstream of said primary expansion valve ( 45 ) and downstream of said first heat exchanger ( 40 ) with respect to refrigerant flow in the defrost mode to a liquid reservoir ( 70 ) disposed in the refrigerant circuit at a second location downstream of said primary expansion valve ( 45 ) with respect to refrigerant flow in the defrost mode.

FIELD OF THE INVENTION

This invention relates generally heat pumps and, more particularly, to arefrigeration circuit for and method of operation of an air source heatpump.

BACKGROUND OF THE INVENTION

Air source heat pumps use ambient outside air as the heat source or heatsink, respectively, for heating or cooling another heat exchange medium.Conventional air source heat pumps include a compressor, a reversingvalve, a first heat exchanger, an expansion device and a second heatexchanger arranged in a refrigerant vapor compression cycle refrigerantcircuit in a manner well-known in the art. Air source heat pumps arecommonly switched between a heating mode and a cooling mode throughselective positioning of the reversing valve. In conventional systems,the first heat exchanger is disposed outdoors and acts as a refrigerantheat rejection heat exchanger, such as a condenser of refrigerant vapor,when the heat pump is operating in the cooling mode and acts as arefrigerant heat absorption heat exchanger, such as a refrigerantevaporator, when the heat pump is operating in the heating mode.Conversely, the second heat exchanger acts as a refrigerant heatabsorption heat exchanger when the heat pump is operating in the coolingmode and acts as a refrigerant heat rejection heat exchanger when theheat pump is operating in the heating mode.

When the air source heat pump is operating in the heating mode,depending upon outdoor ambient conditions, frost can form and build upon the refrigerant tube coils of the first heat exchanger, which acts asa refrigerant heat absorption heat exchanger in the heating mode. It iscustomary practice to periodically defrost the outdoor heat exchangercoil by switching the heat pump to operation in the cooling mode for aperiod of operation sufficient to heat that coil and melt the frostaccumulated thereon, and then switching the heat pump back to operationin the heating mode. When switching the reversing valve from the defrostmode back to the heating mode, an excess of liquid refrigerant migratesto the suction side of the compressor. Therefore, it is customary inconventional air source heat pump systems to include an accumulator inthe refrigerant circuit on the suction side of the compressor upstreamof the suction inlet to the compressor. The accumulator provides areservoir to collect the liquid refrigerant so as to prevent thecarryover of liquid refrigerant into the suction inlet of thecompressor. It is desirable to avoid the introduction of liquidrefrigerant into the compressor as the presence of liquid refrigerant inthe compressor is detrimental to compressor performance. Forconventional large capacity heat pumps, the suction accumulator must besized to handle a significant amount of liquid refrigerant.Consequently, the suction accumulator typically is a high cost item inthe heat pump system. Additionally, the presence of a suctionaccumulator in the refrigerant circuit imparts a refrigerant pressuredrop to the refrigerant passing therethrough to the suction inlet of thecompressor. This added pressure drop adversely impacts the coefficientof performance of the heat pump system.

U.S. Pat. No. 4,843,838 discloses an air-to-air heat pump havingalternate refrigerant lines interconnecting the indoor coil and theoutdoor coil. Each of these refrigerant lines includes a check valve anda float valve. Refrigerant flows through one of these lines duringoperation in the cooling mode and through the other line duringoperation in the heating mode. The usual trap type accumulator is notrequired and is eliminated.

SUMMARY OF THE INVENTION

A heat pump system operable in a cooling mode, a heating mode and adefrost mode includes a refrigerant compressor, a reversing valve, afirst heat exchanger and a second heat exchanger disposed in arefrigerant circuit, and a primary expansion device disposed in therefrigerant circuit intermediate the first heat exchanger and the secondheat exchanger. The reversing valve may be positioned in a firstposition for operation of the heat pump system in the cooling mode andmay be positioned in a second position for operation of the heat pumpsystem in the heating mode.

In an aspect of the invention, the heat pump includes a refrigerantbypass circuit establishing a refrigerant flow path from the refrigerantcircuit at a first location upstream of the primary expansion device anddownstream of the first heat exchanger with respect to refrigerant flowin the defrost mode to a liquid reservoir disposed in the refrigerantcircuit at a second location downstream with respect to refrigerant flowin the defrost mode of the primary expansion valve.

In an embodiment, the second heat exchanger defines a refrigerantcollection chamber comprising the liquid reservoir. The second heatexchanger may a shell and tube heat exchanger having a shell definingthe refrigerant collection chamber and a tube bank heat exchangerdisposed in the refrigerant collection chamber. In an embodiment, theliquid reservoir comprises a refrigerant receiver disposed in therefrigerant circuit intermediate the primary expansion device and thesecond heat exchanger.

The bypass circuit may comprise a bypass refrigerant lineinterconnecting the refrigerant circuit at the first location upstreamof the primary expansion device and downstream of the first heatexchanger with respect to refrigerant flow in the defrost mode inrefrigerant flow communication with the liquid reservoir and a bypassrefrigerant flow control device interdisposed in the refrigerant bypassline. The bypass refrigerant flow control device may comprise a flowcontrol valve having a first position in which the bypass refrigerantline is open to refrigerant flow and a second position in which thebypass refrigerant line is closed to refrigerant flow. In an embodiment,the bypass refrigerant flow control device comprises an openposition/closed position solenoid valve.

In an aspect of the invention, a method is provided for operating a heatpump system during operation in a defrost mode. The heat pump systemincludes a refrigerant compressor, a reversing valve, a first heatexchanger and a second heat exchanger disposed in a refrigerant circuit,and a primary expansion device disposed in the refrigerant circuitintermediate the first heat exchanger and the second heat exchanger. Thereversing valve is positionable in a first position for operation of theheat pump system in the cooling or defrost mode and is positionable in asecond position for operation of the heat pump system in the heatingmode. The method includes the steps of: initiating switching of thereversing valve from its second position into its first position foroperation in the defrost mode; prior to terminating operation in thedefrost mode, passing refrigerant flow from the refrigerant circuitthrough a refrigerant bypass circuit to a liquid refrigerant reservoir;and initiating switching of said reversing valve out of its firstposition. The method may include the further steps of: providing a flowcontrol valve in the bypass refrigerant circuit, the flow control valvehaving an open position in which the bypass refrigerant line is open torefrigerant flow and a closed position in which the bypass refrigerantline is closed to refrigerant flow.

The step of passing refrigerant flow from the refrigerant circuitthrough the refrigerant bypass circuit may comprise opening the flowcontrol valve. The step of initiating the passing of refrigerant flowfrom the refrigerant circuit through the refrigerant bypass circuit byopening the flow control in defrost process may comprise opening theflow control valve when a discharge pressure of the compressor exceeds afirst preselected discharge pressure set point (for example 1650 kPa);or frost factor decreased to 0%, or defrost time (for example 8 minutes)has elapsed. If any one of these three conditions is met, the flowcontrol valve will begin to open.

The step of terminating the passing of refrigerant flow from therefrigerant circuit through the refrigerant bypass circuit may compriseclosing the flow control valve. The step of terminating the passing ofrefrigerant flow from the refrigerant circuit through the refrigerantbypass circuit by closing the flow control valve may comprise the stepof closing the flow control valve after a preset time period haselapsed. The length of the preset time period may range from one secondto 45 seconds or more depending upon the size of the flow control valve.In an embodiment, the preset time period may be about 5 seconds.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the invention, reference will be made tothe following detailed description of the invention which is to be readin connection with the accompanying drawing, where:

FIG. 1 is a schematic diagram illustrating an exemplary embodiment of aheat pump system in accord with the invention illustrating operation ofthe heat pump system in the cooling mode;

FIG. 2 is a schematic diagram illustrating the heat pump system shown inFIG. 1 illustrating operation of the heat pump system in the heatingmode;

FIG. 3 is a schematic diagram illustrating the heat pump system shown inFIG. 1 illustrating operation of the heat pump system at transition fromthe defrost mode into the heating mode;

FIG. 4 is a schematic diagram illustrating another exemplary embodimentof a heat pump system in accord with the invention illustratingoperation of the heat pump system at transition from the defrost modeinto the heating mode;

FIG. 5 is a schematic block diagram illustrating an exemplary embodimentof a method for operating a heat pump system in the defrost mode; and

FIG. 6 is a schematic block diagram illustrating an exemplary embodimentof a method for starting the heat pump system in the heating mode.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to the exemplaryembodiments of the refrigerant heat pump system 10 depicted in FIGS.1-4. In each of these figures, the direction of refrigerant flow isindicated by the arrows flanking the refrigerant circuit lines. Thedepicted heat pump system 10 is of the type commonly referred to as anair source heat pump. However, it is to be understood that the inventionis not limited in application to air source heat pumps.

The heat pump system 10 includes a compressor 20, a reversing valve 30,a first heat exchanger 40 and a second heat exchanger 50 connected inrefrigerant flow communication by a plurality of refrigerant linesforming a closed-loop refrigerant circuit. A primary expansion device 45is disposed in the refrigerant circuit intermediate the first heatexchanger 40 and the second heat exchanger 50. The heat pump system 10may also include an economizer heat exchanger 60 disposed in therefrigerant circuit intermediate the first heat exchanger 40 and thesecond heat exchanger 50. A receiver 70 may also be disposed in therefrigerant circuit intermediate the primary expansion device 45 and thesecond heat exchanger 50.

The reversing valve 30 may comprise a selectively positionable,two-position, four-port valve having a first port 30-1, a second port30-2, a third port 30-3 and a fourth port 30-4. The reversing valve 30is positioned in a first position for coupling the first port 30-1 andthe second port 30-2 in fluid flow communication and for simultaneouslycoupling the third port 30-3 and the fourth port 30-4 in fluid flowcommunication for operation of the heat pump system in the cooling modeand in the defrost mode. The reversing valve 30 is also positioned in asecond position for coupling the first port 30-1 and the fourth port30-4 in fluid flow communication and for simultaneously coupling thesecond port 30-2 and the third port 30-3 in fluid flow communication foroperation in the heating mode. The afore-mentioned respectiveport-to-port couplings established in the first and second positions maybe accomplished internally within the reversing valve 30. Forconvenience of description, the reversing valve will be deemed energizedin cooling mode and not energized in heating mode. The discharge outlet28 of the compressor 20 is connected in fluid flow communication throughrefrigerant line 3 to the first port 30-1 of the reversing valve 30. Anoil separator 26 may be interdisposed in refrigerant line 3 between thecompressor discharge outlet 28 for removing lubricating oil from therefrigerant passing through refrigerant line 3. The suction inlet 22 ofthe compressor 20 is connected in fluid flow communication throughrefrigerant line 5 to the third port 30-3 of the reversing valve 30.

The second port 30-2 of the reversing valve 30 is coupled externally ofthe reversing valve 30 in refrigerant flow communication to the fourthport 30-4 of the reversing valve 30 through a series of refrigerantlines. To operate the heat pump system 10 in a cooling mode, thereversing valve 30 is selectively positioned in its first positionwherein the second port 30-2 of the reversing valve 30 is coupled inrefrigerant flow communication to the fourth port 30-4 of the reversingvalve 30 through refrigerant lines 7, 9A, 11, 13, 15 and 17, which areconnected in series flow relationship with check valves 92 and 94 opento flow and check valves 96 and 98 closed to flow. To operate the heatpump system 10 in a heating mode, the reversing valve 30 is selectivelypositioned in its second position wherein the fourth port 30-4 isconnected in refrigerant flow communication to the second port 30-2 ofthe reversing valve 30 through refrigerant lines 17, 15, 19, 11, 9B and7, which are connected in series flow relationship with check valves 96and 98 open to flow and check valves 92 and 94 closed to flow.

In the cooling mode, the first heat exchanger 40 functions as arefrigerant heat rejection heat exchanger. In the heating mode, thefirst heat exchanger 40 functions as a refrigerant heat absorption heatexchanger. The first heat exchanger 40 is located outdoors, typically onthe roof of or along side a building housing a climate controlled space.One or more fans 42 are disposed in operative association with the firstheat exchanger 40 for passing ambient air through the first heatexchanger in heat exchange relationship with refrigerant passing throughthe refrigerant circuit. In an embodiment, the first heat exchanger 40comprises a heat exchange coil formed of an array of finned tubesthrough which refrigerant passes in heat exchange relationship withambient air passing over the exterior of the tubes and over the surfacesof the fins.

In the cooling mode, the second heat exchanger 50 functions as arefrigerant heat absorption heat exchanger. In the heating mode, thesecond heat exchanger 50 functions as a refrigerant heat rejection heatexchanger. In the depicted embodiment, the second heat exchanger 50 iscoupled through a secondary heat exchange loop to an air side unit 80.The second heat exchanger 50 may also be located exteriorly of theclimate-controlled space, typically outside of the building on the roofof the building or along side the building. In traversing the secondheat exchanger 50, refrigerant from the refrigerant circuit passes inheat exchange relationship with a secondary heat exchange fluid,commonly water or glycol. The secondary heat exchange fluid maytraverses a secondary heat exchange loop wherein the secondary heatexchange fluid passes in heat exchange relationship with air being drawnthrough the air side unit 80 from the climate controlled environment forcooling or heating the air prior to return to the climate controlledenvironment.

In the depicted embodiment, the second heat exchanger 50 comprises ashell and tube heat exchanger having a heat exchange tube bank 52disposed within the interior 55 of the shell 54 of the second heatexchanger 50. The interior 55 of the shell 54 defines a refrigerantcollection chamber. A secondary heat exchange fluid passes through thetube bank 52 in heat exchange relationship with refrigerant from therefrigerant circuit of the heat pump system 10. In operation, theinterior 55 of shell 54 is flooded with refrigerant from the refrigerantcircuit. The refrigerant flows over the exterior of the tubes of thetube bank 52 in heat exchange relationship with the secondary heatexchange fluid passing through the tubes of the tube bank. The heatexchange tube bank may be partially or fully immersed in therefrigerant. In the depicted embodiment, the tube bank 52 comprises afirst heat transfer module of a secondary heat exchange loop throughwhich the secondary heat exchange fluid circulates. The secondary heatexchange loop includes a second heat transfer module which comprises aheat exchanger tube coil 82 of the air side unit 80, such as forexample, but not limited to, an air handling unit or a fan coil unit,wherein the secondary heat exchange fluid passes through the heatexchanger tube coil 82 in heat exchange relationship with indoor airdrawn from the climate-controlled space. In passing over the heatexchanger tube coil 82, the indoor air is cooled during operation of theheat pump system 10 in the cooling mode and is heated during operationof the heat pump system 10 in the heating mode.

As depicted in the embodiment of the heat pump system 10 illustrated inFIGS. 1-4, the heat pump system 10 may include an economizer heatexchanger 60. In the depicted embodiment, the economizer heat exchanger60 may comprise a refrigerant-to-refrigerant heat exchanger having afirst refrigerant circuit branch 62 and a second refrigerant circuitbranch 64 disposed in heat exchange relationship. The first refrigerantcircuit branch 62 comprises a portion of the refrigerant line 11. Thesecond refrigerant circuit branch 64 comprises a portion of economizerrefrigerant line 21 which taps into the refrigerant line 11 at alocation upstream of the first refrigerant circuit branch 62 andprovides a refrigerant flow passage from that point to an intermediatepressure port 24 of the compressor 20 through which refrigerant vapormay be injected into an intermediate pressure, that is a pressureintermediate the suction pressure and the discharge pressure, chamber ofthe compressor 20. A secondary expansion device 65 is disposed in theeconomizer refrigerant line 21 at a location upstream with respect torefrigerant flow of the second refrigerant circuit branch 64.

The economizer 60 is typically in operation in both the cooling mode andthe heating mode, but generally not operated in the defrost mode. Whenthe economizer is in operation, a portion of the liquid refrigerantpassing through refrigerant line is diverted to flow through refrigerantline 21 and to traverse the secondary expansion device 65. The secondaryexpansion device 65 functions to expand refrigerant passing therethroughfrom a higher pressure, higher temperature refrigerant liquid to a lowerpressure, lower temperature refrigerant vapor or liquid/vapor mix. Thelower pressure, lower temperature refrigerant vapor or vapor/liquid mixpasses through the second refrigerant circuit branch 64 in heat exchangerelationship with the higher pressure, higher temperature refrigerantliquid passing through the first refrigerant circuit branch 62 wherebythe refrigerant liquid is further cooled prior to traversing the primaryexpansion device 45 and the refrigerant vapor passing through the secondrefrigerant circuit branch 64 is heated prior to injection into anintermediate pressure stage of the compression process. The primaryexpansion valve 45 is disposed in refrigerant line 11 downstream withrespect to refrigerant flow of the first refrigerant circuit branch 62.

Referring now to FIG. 1 in particular, in operation of the heat pumpsystem 10 in the cooling mode, hot, high pressure refrigerant vapordischarging from the compressor 20 through refrigerant line 3 passesthrough the reversing valve 30 from the first port 30-1 to the secondport 30-2, thence through refrigerant line 7, thence through the firstheat exchanger 40, thence through refrigerant line 9A, thence throughrefrigerant line 11 traversing the economizer heat exchanger 60 and theprimary expansion valve 45, thence through refrigerant line 13, thencethrough refrigerant line 15, traversing the receiver 70 and the secondheat exchanger 50, thence through refrigerant line 17 to the reversingvalve 30. When the heat pump system 10 is operating in the cooling mode,the secondary heat transfer medium is passed through the second heatexchanger 50 in heat exchange relationship with the refrigerant withinthe second heat exchanger 50, whereby refrigerant is evaporated and thesecondary heat transfer medium is cooled. In the cooling mode ofoperation, the refrigerant leaving the second heat exchanger 50 andpassing through refrigerant line 17 consists of refrigerant vapor withlittle or no liquid refrigerant carryover. The refrigerant vapor passesfrom refrigerant line 17 into the fourth port 30-4 of the reversingvalve 30 and out the third port 30-3 of the reversing valve 30 into andthence through the refrigerant line 5 to return to the compressor 20through the suction inlet 22 to the compressor 20.

Referring now to FIG. 2 in particular, in operation of the heat pumpsystem 10 in the heating mode, hot, high pressure refrigerant vapordischarging from the compressor 20 through refrigerant line 3 passesthrough the reversing valve 30 from the first port 30-1 to the fourthport 30-4, thence through refrigerant line 17, thence through the secondheat exchanger 50, thence through refrigerant lines 15 and 19,traversing the receiver 70, thence through refrigerant line 11traversing the economizer heat exchanger 60 and the primary expansionvalve 45, thence through refrigerant line 9B, thence through the firstheat exchanger 40 in heat exchange relationship within ambient outdoorair, thence through refrigerant line 7 to the reversing valve 30. Whenthe heat pump is operating in the heating mode, the first heat exchanger40 functions as a refrigerant evaporator, whereby the refrigerantleaving the first heat exchanger 40 and passing through refrigerant line7 consists of refrigerant vapor with little or no liquid refrigerantcarryover. The refrigerant vapor passes from refrigerant line 7 into thesecond port 30-2 of the reversing valve 30 and out the third port 30-3of the reversing valve 30 into and thence through the refrigerant line 5to return to the compressor 20 through the suction inlet 22 to thecompressor 20.

When the heat pump system 10 is operating in the heating mode, thesecond heat exchanger 50 receives hot, high pressure refrigerant vapordischarged from the compressor. As the secondary heat transfer medium ispassed through the second heat exchanger 50 in heat exchangerelationship with the refrigerant vapor within the second heat exchanger50, the refrigerant vapor is condensed and the secondary heat transfermedium is heated. Thus, in the heating mode of operation, the secondheat exchanger 50 is operating as a refrigerant heat rejection heatexchanger, that is, a refrigerant condenser. In the heating mode ofoperation, the refrigerant leaving the second heat exchanger 50 andpassing through refrigerant line 15 consists of liquid phaserefrigerant.

In heat pump applications in temperate climates, depending upon ambientconditions, such as outdoor air temperature and humidity, duringoperation of the heat pump system in the heating mode, frost may form onthe refrigerant conveying heat exchange coils of the first heatexchanger 40. Therefore, when operating the heat pump system in theheating mode, it is necessary to periodically interrupt operation in theheating mode and operate the heat pump system in a cooling mode for alimited period in order to defrost the heat exchange coils of the firstheat exchanger 40. Switching into the defrost mode may be doneautomatically after a preset time period of operation in the heatingmode or it may be done in response to a frost sensor, such as forexample, but not limited to, a coil temperature sensor 41 operativelyassociated with the heat exchange coils of the first heat exchanger 40,or in response to an operating parameter. Termination of the defrostmode and switch back into the heating mode may also be doneautomatically after a preset time period of operation in the defrostmode or it may be done in response to a frost sensor operativelyassociated with the heat exchange coils of the first heat exchanger 40or in response to an operating parameter.

Referring now to FIGS. 3 and 4 in particular, the heat pump system 10 isequipped with a refrigerant bypass line 23 which taps into refrigerantline 11 at a location downstream with respect to refrigerant flow of theeconomizer 60 and upstream with respect to refrigerant flow of theprimary expansion valve 45. In the exemplary embodiment of the heat pumpsystem 10 depicted in FIG. 3, the refrigerant bypass line 23 opens intothe interior chamber of the receiver 70. In the exemplary embodiment ofthe heat pump system 10 depicted in FIG. 4, the refrigerant bypass line23 opens into the refrigerant collection chamber 55 defined by theinterior of the shell 54 of the shell and tube heat exchanger 50. Abypass flow control device 75 is disposed in the refrigerant bypass line23. The bypass flow control device 75 may be selectively positioned inat least a first open position and a second closed position. With thebypass flow control device 75 positioned in its open position,refrigerant flows through the refrigerant bypass line 23 to either thereceiver 70 or the shell and tube heat exchanger 50, bypassing theprimary expansion device 45. With the bypass flow control device 75positioned in its closed position, refrigerant flow through therefrigerant bypass line 23 is blocked and the refrigerant flowingthrough refrigerant line 11 continues on to pass through the primaryexpansion device 45. In an embodiment, the bypass flow control device 75may comprise, for example but not limited to, a two-position on/offsolenoid valve.

During steady state operation of the heat pump system 10 in either theheating mode or the cooling/defrost mode, the bypass flow control device75 is positioned in a closed position. Upon termination of operation inthe defrost mode, the reversing valve 30 is repositioned from thecooling/defrost mode position into the heating mode position. Prior totransition of the heat pump system 10 from the defrost mode into theheating mode by repositioning the reversing valve 30, the bypass flowcontrol device 75 is positioned in an open position to divert liquidrefrigerant flowing through refrigerant line 11 through the bypassrefrigerant line 23 directly into either the receiver 70 or therefrigerant collection chamber 55 of the shell and tube heat exchanger50 without traversing the primary expansion valve 45. Because therefrigerant flowing through the bypass refrigerant line 23 does not passthrough the primary expansion valve 45, this refrigerant remains in theliquid phase. By diverting the refrigerant flow through bypassrefrigerant line 23 to collect in either of the receiver 70 or thechamber 55 of the shell and tube heat exchanger 50 during the transitionout of defrost, liquid refrigerant is permitted to pass from the firstheat exchanger 40 directly into the receiver 70 or the chamber 55 of theshell and tube heat exchanger 50. In this manner, liquid refrigerantdrains from the first heat exchanger 40 so that upon entering back inthe heating mode of operation at the end of the transition from thedefrost mode, little or no liquid refrigerant will be resident in thefirst heat exchanger 40, thereby minimizing or eliminating the potentialfor liquid phase refrigerant to be introduced into the compressor 20through the suction inlet 22 when operation upon commencement ofoperation in the heating mode.

Typically, the reversing valve 30 remains positioned in the heating modeposition after a heating mode cycle is completed and the compressor 20is powered off, for example if heating demand has been satisfied or inthe event of an emergency electrical power loss. With the reversingvalve 30 remaining in the heating position, when restarting thecompressor 20, liquid refrigerant in the heat exchange coil of the firstheat exchanger 40 would flush into the compressor 20 because the heatexchange coil of the heat exchanger 40 is connected in fluidcommunication with the suction port 22 of the compressor 20 when thereserving valve 30 is positioned in the heating mode. Since theintroduction of a significant amount of liquid refrigerant into thecompressor 20 with the suction inlet 22 would be detrimental to thecompressor 20, in the heat pump system 10 a flow control valve 43 and aflow check valve 47, disposed in a parallel arrangement, areinterdisposed in refrigerant line 9 between the heat exchange coil ofthe first heat exchanger 40 and the intersection of refrigerant branchlines 9A and 9B. Thus, in reference to the heating mode, the flowcontrol valve 43 and the flow check valve 47 are disposed upstream withrespect to refrigerant flow through the circuit of the first heatexchanger 40 and downstream with respect to refrigerant flow of theprimary expansion device 45. The flow check valve 47 provides a bypasscircuit for refrigerant flow to bypass the flow control valve 43 whenthe flow control valve 43 is closed and the refrigerant is flowing fromthe first heat exchanger 40 through refrigerant line 9 as in the coolingor defrost mode. When refrigerant is flowing into the first heatexchanger 40 from refrigerant line 9 and through the flow control valve43 as in the heating mode, the flow check valve 47 is inherently closedto flow.

When the compressor 20 restarts in the heating mode, for example aftercompletion of a defrost mode, the flow control valve 43, which may be asolenoid valve, such as a solenoid valve having an open position and aclosed position, is positioned closed so that no liquid refrigerant willflow into the heat exchange coil of the first heat exchanger 40. Thusthe refrigerant pressure, and therefore the saturated evaporationtemperature, SET, of the refrigerant within the heat exchange coil ofthe heat exchanger 40 will decrease. Consequently, the liquidrefrigerant within the heat exchange coil will start to evaporate intorefrigerant vapor.

When the refrigerant liquid is almost completely evaporated, therefrigerant pressure and, correspondingly, the saturate evaporationtemperature will further decrease. Once the temperature differencebetween ambient temperature and the saturated evaporation temperaturereaches a preset temperature differential set point, such as for example12 degrees Kelvin (21.6 degrees Fahrenheit), the flow control valve 43is opened to allow the refrigerant from refrigerant line 9 to pass intoand through the heat exchange coil of the heat exchanger 40. When theflow control valve 43 is initially opened, there will generally beliquid refrigerant carried mixed in the refrigerant vapor passing fromthe heat exchange coil of the first heat exchanger 40 into thecompressor 20 through the suction inlet 22. Because the compressor 20 isoperating at a low capacity at the beginning of compressor startup, therelatively small amount of liquid refrigerant carryover into thecompressor 20 for a short period after opening the flow control valve 43will have no detrimental effect on the compressor 20 and is relativelysafe. After compressor restarting, the reversing valve starts toreposition to the heating mode from the defrost mode only when therefrigerant pressure differential between the discharge pressure, PD,and the suction pressure, PS, exceeds a preset pressure differential,such as for example 350 kPa (kilopascals), substantially reduces therisk of liquid carryover upon restarting. If the compressor 20 istripped in response to an alarm condition while operating in the defrostmode, the compressor 20 will be restarted in the defrost mode afterreset.

Referring now to FIG. 5, there is depicted in block diagram format thesteps of a method for operating the heat pump system 10 in a defrostmode of operation. In the embodiment of the method as depicted in FIG.5, the period of operation of the heat pump system 10 in the defrostmode is initiated either in response to a frost accumulation sensor,such as for example, but not limited to, the coil temperature sensor 41,for example a thermistor, operatively associated with the heat exchangercoil of the outdoor heat exchanger 40 or in response to a lowrefrigerant suction pressure override being initialized. At 102-1, thesystem controller (not shown) monitors the output of the coiltemperature sensor 41 and determines the magnitude of a coil frostfactor based upon the output received from the coil temperature sensor.If the frost factor reaches 100%, the system controller, at 104, willfirst energize the solenoid valve 75 and then initiate the repositioningof the reversing valve 30 from a heating mode position to a defrost modeposition. At 102-2, the system controller monitors the temperature ofthe refrigerant entering the compressor 20 through the suction inlet 22and determines whether the suction saturation temperature is less than alower limit set point suction temperature, such as for example minus26.4 degrees Celsius. If the suction saturation temperature is less thanthe lower limit set point suction temperature for a preset period oftime, such as for example fifty-five seconds, the system controllerwill, at 104, will first energize the refrigerant bypass flow controlvalve 75 and then initiate the repositioning of the reversing valve 30from a heating mode position to a defrost mode position.

With the reversing valve 30 positioned in the defrost mode, refrigerantwill flow through the refrigerant circuit of the heat pump system 10 asdescribed with respect to the cooling mode and illustrated in FIG. 1.During the defrost mode, the system controller continuously monitors:the coil temperature via temperature sensor 41, the elapsed time indefrost, for example by means of a software program, and the refrigerantpressure at the compressor via a pressure sensor, such as for example apressure transducer, for example by means of a pressure sensingtransducer (not shown) operatively associated with the compressorrefrigerant discharge line for sensing the refrigerant pressure at thecompressor discharge. At 106-1, the system controller compares thesensed compressor refrigerant discharge pressure with an upper limit setpoint refrigerant discharge pressure, such as for example, but notlimited to, 1650 KPa (kilopascals). At 106-2, the system controllercompares the sensed coil temperature to a set point coil temperature,such as for example 14 degrees Celsius, or to a temperature valuecalculated by the controller based upon a preprogrammed function andindicative of a zero frost factor. At 106-3, the system controllercompares the elapsed time in the defrost mode with a preprogrammedmaximum time period permitted in defrost.

If any one of the three conditions is met, that is if the sensedcompressor discharge temperature has reaches the upper limit set point,or the sensed coil temperature has reached the coil temperature setpoint, or the elapsed time in defrost has reached the maximum time limitfor the defrost mode, the system controller, at 108, opens the solenoidopens the refrigerant bypass flow control valve 75. With the refrigerantbypass flow control valve 75 now open, refrigerant flows through thebypass refrigerant line 23 directly to a liquid reservoir, namely eitherto the receiver 70 or to the refrigerant collection chamber 55 of thesecond heat exchanger 50, bypassing the primary expansion valve 45. Whenthe bypass flow control valve 75 is open, the primary expansion valve 45may be maintained at an initial fixed opening set upon entry into thedefrost mode or reset upon opening of the bypass flow control valve 75and maintained at a new fixed opening. At 110, the system controllerwill monitor the time elapsed since the bypass flow control valve 75 wasopened and determine when the bypass flow control valve has been openfor a preprogrammed time period, such as for example 5 seconds, at whichpoint the system controller will at 112 closed the refrigerant bypassflow control valve 75 and at 114 initiate repositioning of the reversingvalve 30 out of the defrost mode.

When compressor 20 is in standby mode or in delay status, the reversingvalve 30 will maintain the original status in which the compressor 20was most recently operating, unless an accidental or emergencyelectrical power loss has occurred when the compressor was in idle.Thus, under normal circumstances, if the compressor 20 stopped in theheating mode, the reversing valve 30 will remain in the heating modeposition, and if the compressor 20 stopped in the cooling or defrostmode, the reversing valve 30 will keep remain in the cooling/defrostmode position. If the compressor 20 restarts in the cooling mode,refrigerant liquid carryover into the compressor 20 is not a concernbecause sufficient capacity exists within the interior 55 defined by theshell 54 of the second heat exchanger to collect the liquid refrigerantand prevent the liquid refrigerant from proceeding directly intocompressor directly.

Referring now to FIG. 6, if the system controller, at 301, anticipatesthat the compressor 20 restarts in the heating mode, to prevent all theliquid refrigerant in the heat exchange coil of the first heat exchanger40 from flushing through refrigerant line 7 and refrigerant line 5 tothe suction inlet 22 and directly into the compressor 20, the systemcontroller will first, at 302, close the solenoid valve 43 so that norefrigerant fluid can enter the heat exchange coil of the first heatexchanger 40. The system controller will then start the compressor 20.With the compressor 20 now running at 303 and the solenoid valve 43closed, the pressure and saturated temperature of refrigerant within theheat exchange coil of the first heat exchanger 40 will drop downgradually. Thus, the liquid refrigerant inside of heat exchange coilwill evaporate into vapor by absorbing heat from the ambient air passingover the heat exchange coil. As the refrigerant pressure and saturatedtemperature continue to drop lower and lower, the system controller willmonitor the saturated suction temperature and, at 304, compare thesaturated suction temperature to the ambient air temperature. Once atemperature differential defined as the ambient air temperature minusthe saturated suction temperature exceeds a set point temperaturedifference, such as for example 12 degrees Celsius, the systemcontroller, at 305, will open the solenoid valve 43 so that the expandedrefrigerant from expansion device 45 may now pass through the valve 43into the heat exchange coil of the first heat

Operation of the heat pump system 10 in accordance with theafore-described method for operation during the defrost mode ensuresthat all or substantially all of the liquid refrigerant resident in thecoil of first (outdoor) heat exchanger 40 and the various refrigerantcircuit lines at termination of the defrost mode drains into the eitherthe receiver 70 or the refrigerant collection chamber 55 of the secondheat exchanger 50. Therefore, little or no liquid refrigerant remains inthe coil of the first heat exchanger 40 or the refrigerant circuit linesthat could possibly be carried over into the suction inlet 22 of thecompressor 20 upon switching from the defrost mode into the heatingmode. As a result, the accumulator 74 may be substantially reduced insize in comparison to the typical size of an accumulator of aconventional prior art heat pump system of comparable capacity. In someapplications, with the heat pump system of the present invention, theaccumulator 74 may even be eliminated from the refrigerant circuit suchas depicted in the exemplary embodiment of the heat pump system 10depicted in FIG. 4.

The method of operation in the defrost mode has been describedhereinbefore with respect to a heat pump system equipped with a frostsensor, such as, for example, the coil temperature sensor 41 operativelyassociated with the first (outdoor) heat exchanger 40. It is to beunderstood, however, that the heat pump system 10 of the invention maybe also operated in a defrost mode entered into at periodic timeintervals of operation in the heating mode and extending for a limitedperiod of time of operation in the defrost mode before returning to theheating mode, without departing from the spirit of the method ofoperation of the present invention.

The terminology used herein is for the purpose of description, notlimitation. Specific structural and functional details disclosed hereinare not to be interpreted as limiting, but merely as basis for teachingone skilled in the art to employ the present invention.

While the present invention has been particularly shown and describedwith reference to the exemplary embodiments as illustrated in thedrawing, it will be recognized by those skilled in the art that variousmodifications may be made without departing from the spirit and scope ofthe invention. Those skilled in the art will also recognize theequivalents that may be substituted for elements described withreference to the exemplary embodiments disclosed herein withoutdeparting from the scope of the present invention. Therefore, it isintended that the present disclosure not be limited to the particularembodiment(s) disclosed as, but that the disclosure will include allembodiments falling within the scope of the appended claims.

1. A heat pump system operable in a cooling mode, a heating mode and adefrost mode and including a refrigerant compressor, a reversing valve,a first heat exchanger and a second heat exchanger disposed in arefrigerant circuit, and a primary expansion device disposed in therefrigerant circuit intermediate said first heat exchanger and saidsecond heat exchanger, said reversing valve being positionable in afirst position for operation of said heat pump system in the coolingmode or defrost mode and being positionable in a second position foroperation of said heat pump system in the heating mode, said heat pumpcharacterized by: a refrigerant bypass circuit establishing arefrigerant flow path from the refrigerant circuit at a first locationupstream of said primary expansion device and downstream of said firstheat exchanger with respect to refrigerant flow in the defrost mode to aliquid reservoir disposed in the refrigerant circuit at a secondlocation downstream with respect to refrigerant flow in the defrost modeof said primary expansion valve.
 2. The heat pump system of claim 1further characterized in that said second heat exchanger defines arefrigerant collection chamber comprising said liquid reservoir.
 3. Theheat pump system of claim 2 further characterized in that said secondheat exchanger comprises a shell and tube heat exchanger having a shelldefining the refrigerant collection chamber and a tube bank heatexchanger disposed in the refrigerant collection chamber.
 4. The heatpump system of claim 1 further characterized in that said liquidreservoir comprises a refrigerant receiver disposed in the refrigerantcircuit intermediate said primary expansion device and said second heatexchanger.
 5. The heat pump system of claim 1 further characterized inthat said bypass circuit comprises: a bypass refrigerant lineinterconnecting the refrigerant circuit at the first location upstreamof said primary expansion device and downstream of said first heatexchanger with respect to refrigerant flow in the defrost mode inrefrigerant flow communication with said liquid reservoir; and a bypassrefrigerant flow control device interdisposed in said refrigerant bypassline.
 6. The heat pump system of claim 5 further characterized in thatsaid bypass refrigerant flow control device comprises a flow controlvalve having a first position in which said bypass refrigerant line isopen to refrigerant flow and a second position in which said bypassrefrigerant line is closed to refrigerant flow.
 7. The heat pump systemof claim 5 further characterized in that said bypass refrigerant flowcontrol device comprises an open position/closed position solenoidvalve.
 8. The heat pump system of claim 1 further characterized by: aflow control valve disposed in the refrigerant circuit upstream of thesaid first heat exchanger and downstream of primary expansion devicewith respect to refrigerant flow through the circuit in a heating mode;and a flow check valve disposed in the refrigerant circuit in parallelrelationship with said flow control.
 9. A method of operating the heatpump system during a defrost mode, the heat pump including a refrigerantcompressor, a reversing valve, a first heat exchanger and a second heatexchanger disposed in a refrigerant circuit, and a primary expansiondevice disposed in the refrigerant circuit intermediate said first heatexchanger and said second heat exchanger, said reversing valve beingpositionable in a first position for operation of said heat pump systemin the cooling or defrost mode and being positionable in a secondposition for operation of said heat pump system in the heating mode,said method characterized by the steps of: initiating switching of saidreversing valve from its second position into its first position foroperation in the defrost mode; prior to terminating operation in thedefrost mode, passing refrigerant flow from the refrigerant circuitthrough a refrigerant bypass circuit to a liquid reservoir; andinitiating switching of said reversing valve out of its first position.10. The method of claim 9 further characterized by: providing a flowcontrol valve in said bypass refrigerant circuit, said flow controlvalve having an open position in which said bypass refrigerant line isopen to refrigerant flow and a closed position in which said bypassrefrigerant line is closed to refrigerant flow; and in that the step ofpassing refrigerant flow from the refrigerant circuit through saidrefrigerant bypass circuit comprises opening said flow control valve.11. The method of claim 10 further characterized in that the step ofpassing refrigerant flow from the refrigerant circuit through saidrefrigerant bypass circuit by opening said flow control comprisesopening said flow control valve in defrost mode when a dischargepressure of said compressor exceeds a first preselected dischargepressure set point.
 12. The method of claim 10 further characterized inthat the step of passing refrigerant flow from the refrigerant circuitthrough said refrigerant bypass circuit by opening said flow controlcomprises opening said flow control valve in defrost mode when a frostfactor for the first heat exchanger drops to 0%.
 13. The method of claim10 further characterized in that the step of passing refrigerant flowfrom the refrigerant circuit through said refrigerant bypass circuit byopening said flow control comprises opening said flow control valve indefrost mode when the time elapsed in a defrost mode reaches eightminutes.
 14. The method of claim 10 further characterized by the step ofterminating the passing of refrigerant flow from the refrigerant circuitthrough said refrigerant bypass circuit by closing said flow controlvalve.
 15. The method of claim 14 further characterized in that the stepof terminating the passing of refrigerant flow from the refrigerantcircuit through said refrigerant bypass circuit comprises closing saidflow control valve upon expiration of said preselected period of time.16. The method as recited in claim 15 further characterized in that saidpreselected period of time ranges from one second to forty-five seconds.17. The method as recited in claim 16 further characterized in that saidpreselected period of time is about five seconds